Synopsis: ENSO-neutral conditions are expected to prevail during the northern summer (June-August), in spite of recent increases in SST anomalies associated with strong Madden-Julian Oscillation (MJO) activity.

Surface and subsurface water temperatures increased substantially in the eastern equatorial Pacific during April, associated with the arrival of the downwelling phase of a strong oceanic Kelvin wave. Sea surface temperature (SST) anomalies increased by more than 2°C in the extreme eastern equatorial Pacific during April (Fig. 1, bottom), and by the end of the month, positive equatorial SST anomalies greater than +0.5°C (~0.9°F) were observed in most areas from Indonesia eastward to the South American coast (Fig. 1, top). The increase in SST anomalies in the eastern equatorial Pacific during April was reflected by an increase in the SST anomalies in the Niño 3 and Niño 1+2 regions(Fig. 2) and by an increase in the upper-ocean heat content in the eastern half of the equatorial Pacific (Fig. 3). Subsurface cooling and a decrease in upper-ocean heat content have been evident in the central equatorial Pacific, associated with the upwelling phase of the Kelvin wave (Fig. 3). This cooling is expected to propagate eastward, reaching the eastern equatorial Pacific during May. Thus, the effects of the warming along the west coast of South America should be brief.

Cloudiness, precipitation and low-level winds displayed considerable week-to-week variability during the month, associated with strong MJO activity (Fig. 4 and Fig. 5). During the first ten days of April enhanced precipitation (negative OLR anomalies) was observed over Indonesia (Fig. 4 top left panel), while stronger-than-average easterlies prevailed over the central equatorial Pacific (Fig. 4, top right panel). The enhanced precipitation moved eastward into the western tropical Pacific during mid-April, (Fig. 4, middle left panel) accompanied by anomalous westerly low-level winds over the extreme western equatorial Pacific (Fig. 4, middle right panel). However, during the last ten days of April the OLR anomalies and low-level wind anomalies weakened over the central equatorial Pacific and drier-than-average conditions developed over Indonesia (Fig. 4 bottom panels). Continued strong week-to-week variability in the patterns of tropical atmospheric circulation and precipitation is likely during May.

A majority of the statistical and coupled model forecasts indicate that ENSO-neutral conditions will prevail during the northern summer (June-August). The spread in the forecasts indicates increasing uncertainty during the last half of 2005.

This discussion is a consolidated effort of NOAA and its funded institutions. Weekly updates for SST, 850-hPa wind, OLR and features of the equatorial subsurface thermal structure are available on the Climate Prediction Center web page at http://www.cpc.ncep.noaa.gov (Weekly Update). Forecasts for the evolution of El Niño/La Niña are updated monthly in the Forecast Forum section of CPC's Climate Diagnostics Bulletin. The next ENSO Diagnostics Discussion is scheduled for 9 June 2005. To receive an e-mail notification when the monthly ENSO Diagnostic Discussions are released, please send an e-mail message: ncep.list.enso-update@noaa.gov.